Even during a routine oil change a great mechanic (or shop) should ADVISE you of anything that's wrong or requires attention. I already knew that something critical needed attention, but was assured that it was ok. After doing other repairs and test-driving it they called me confirming what I already knew.
After I gave a go-ahead on all work, they still managed to omit one small, but important repair (didn't charge for it, but still).
I also later found out that another local mechanic would have charged 40% less and done a better job advising. I.e. TransColonial advised that all 4 struts needed replaced, whereas the other shop said that only the 2 rear are critical (and quoted a much lower price).
I found my new mechanic (and TransColonial isn't it).